Field of the Invention
The present invention relates to a bus architecture communications scheme for enabling communications between a plurality nodes or devices in a computer system. More particularly, the present invention relates to an interface between two devices connected via the bus in which binary signals in the form of NRZ data signals are transmitted through an isolation barrier disposed between the two devices.
Computer devices within a given computer system, such as a microprocessor, a disk drive, a CRT, a printer and the like, need the ability to convey signals between themselves. In the electronics and computer fields, this has been accomplished by means of a bus which comprises a plurality of transmission wires and acts as a communications path for interconnecting several devices in the system. Each device of the system need only plug into the bus to be theoretically connected to each of the other devices in the system. In order to communicate with other devices attached to the bus, each must be equipped with hardware such as transmitting and receiving circuitry compatible with the communications protocol implemented for the bus. However, due to the small signal voltages and currents driven on the bus by each of the above-circuits, a means of electrical or galvanic isolation must be implemented between the circuits. This isolation reduces system ground loop currents and prevents ground drops from interfering with the signal transmission. In conventional bus architectures, one way to electrically isolate the circuitries connected to the bus is to place a pulse transformer in a module at the end of a cable (comprising the bus) to be attached to an associated device.
However, a major drawback in the use of a pulse transformer implemented as the isolation barrier is that it acts as a high pass filter due to the fact that pulse transformers only AC couple the input to the output. If the transmitter drives a high signal on the bus for a prolonged period, the signal as seen by the receiver begins to fall due to the high pass filtering. Hence, not all the data in a broad band spectrum NRZ data signal is transmitted through the narrow band channel created by the pulse transformer because of the unrecoverable loss of the lower band data.
Currently, there are several techniques of passing broad band NRZ data signals through a narrow band channel. One such technique consists of analog and digital modulation wherein a carrier wave is used to transmit broad band data on a specific frequency. However, this technique is relatively complicated, requiring a significant amount of hardware which increases costs and generally requires circuits not available in standard cell gate array technology.
Another prior art transmission technique provides for encoding of the NRZ data signal at the transmitting end of the cable and decoding of the data signal at the receiving end of the cable. This is typically performed by Manchester, 4B5B, 8B10B, etc. encoding of the data signal in which a balanced code is maintained so as to provide an essentially constant DC level. For example, Manchester is a bit level balanced code in which the DC level from bit to bit does not change, while 4B5B and 8B10B are byte level balanced codes in which the DC level of a byte changes by less than 10 to 20%. However, such encoding techniques increase the bandwidth of the NRZ data signal. As an example, when a 50 Megabit Manchester encoded signal is transmitted across the isolation barrier, the bandwidth of the signal is in the range of 20 to 50 MHz rather than the original NRZ spectrum of DC to 25 MHz. Furthermore, a significant amount of electronic hardware is required to implement these coding techniques, thereby increasing the cost and reducing the performance (i.e. higher bit error rates) of the overall bus architecture.
In addition to the foregoing, another drawback in the use of prior art transmission methods and apparatus is that they are based upon typical bus architecture schemes, such as SCSI, Ethernet and ADB, which do not provide for multi-speed signal transmissions on an upward compatible bus where the capabilities of faster devices connected to such a multi-speed bus can be realized.
Accordingly, it is an object of the present invention to provide a method and apparatus for the transmission of NRZ data signals across an isolation barrier disposed between adjacent devices on a bus.
Another object of the present invention is to provide a method and apparatus for transmitting broad band NRZ data signals across a pulse transformer type isolation barrier having a narrow band channel.
A further object of the present invention is to implement the method and apparatus of the present invention in a multi-speed bus architecture scheme using digital standard cell or gate array technology so as to transmit NRZ data signals across a bi-directional interface between adjacent devices coupled to the bus.